Continuous molding of thermoplastic resin

ABSTRACT

Molten thermoplastic resin, e.g., polyethylene is forced into a pattern on the surface of a rotating roll to form a web of the resin, the resin being forced into the pattern through a die having a zigzag upstream edge.

0 t t l Unit States Patent 1 M 9 1 Fields [451 Jan, 11, 11921111;

[54] CON'IWIJOUS MOLDING 0F 2,624,914 1/1953 Rhodes ..264/216 x T PL2,678,493 5/1954 Edmunds, .Ir. ..l8/l5 S X H RMU ASTIC RESIN 3,505,1574/1970 Fields et al ..264/l67 X [72] Inventor: Reuben Thomas Fields,Wilmington, Del.

FOREIGN PATENTS OR APPLICATIONS [73] Assignee: 1E. 11. du Pont deNemours and Company,

Wilmington, DeL 410,168 5/1934 Great Britain 18/15 F 579,245 7/1958Italy ..18/15 F [22] Filed: June 10, 1970 N h: 4 1 PrimaryExaminer-Robert L. Spicer, Jr. [211 App] 0 66 Att0rneyMartin R. Levy[52] 10.8. C1 ..425/223, 264/216, 425/224, [57 ABSTRACT [51] 1m ClMolten thermoplastic resin, e.g., polyethylene is forced into a [58]Fiefid 9 10 pattern on the surface of a rotating roll to form a web ofthe 7 resin, the resin being forced into the pattern through a diehaving a zigzag upstream edge. [56] Rem-ewes Cited 8 Claims, 5 DrawingFigures UNITED STATES PATENTS 2,052,695 9/1936 Chiverton ..l8/l5 F XPATENIEU JAN18|872 3,635,631

FIG- 4B FIG-4A INVENTOR REUBEN T. FIELDS 1 CONTINUOUS MOLDING OFTHERMOPLASTIC RESIN This invention relates to the continuous molding ofmolten thermoplastic resin into a web as described in U.S. applicationSer. No. 619,994, now U.S. Pat. No. 3,515,778, filed Mar. 2, 1967.

in the process there discussed, pressurized molten thermoplastic resinis passed, substantially free of pressure drop, along a laterallyconfined path which terminates at an outlet which is in pressure-sealrelation with a relatively cool patterned solid surface moving acrossthe outlet, whereby the molten resin is forced into the pattern of thesurface to continuously mold a patterned web which is removed from thesurface at a point remote from said outlet. Apparatus for carrying outthis process is also discussed in U.S. application Ser. No. 619,994filed Mar. 2, 1967 and includes structure such as a patterned rolldefining the moving patterned solid surface, a source of pressurizedmolten thermoplastic resin, die means communicating between the sourceand the surface and having its outlet in pressure-seal relation with thelatter for receiving resin from the source and directing the resinthrough its outlet without any substantial loss of pressure against thepatterned surface, thus the pressure upon the resin forces it into thepattern of the surface to continuously mold a patterned web of resinwhich is then transported away from the die means by the movement of thesurface, and means for cooling the surface so as to chill the resincoming into contact therewith.

The pressure upon the molten resin within the die means is present atthe interface between the outlet of the die means and the patternedsurface and is available for a sufficient duration to cause evenintricate patterns to be filled at high speeds of movement of thesurface. Surprisingly, these results are not precluded by the chillingof the molten thermoplastic resin coming into contact with the patternedsurface. Further surprising is the fact that thin films on the order of0.001 to 0.005-inch inch thick, can be molded despite chilling of themolten resin in contact with the patterned surface.

In another embodiment of that apparatus, the same pressureseal relationbetween the outlet of the die means and patterned surface is maintained,except that the outlet includes a wedge-shaped passage extending in thedirection of movement of the patterned surface and having its baseformed by the moving patterned surface. This wedge-shaped passagereceives molten resin by drag flow of the resin across the path of theresin within the die means by the patterned surface, and, in turn,forces the received resin into the pattern of the surface. This dragflow augments the pressure exerted upon the resin in passing along thelaterally confined path. For simple patterns, drag flow pressure aloneis sufficient to obtain pattern filling provided that sufficient moltenresin is supplied to the wedge-shaped passage. The wedge-shaped passagealso has the effect of increasing the area and time of contact betweenmolten thermoplastic resin and the relatively cool patterned surfaceduring molding, an effect which would be expected to increase moldingdifficulty. Surprisingly, however, this embodiment, because of thepressure created in the wedge-shaped passage, enables more intricatepatterns and even higher production speeds to be attained than with thefirst described apparatus embodiment.

it has been found, however, that when filling screen-type patterns inthe patterned roll having intersecting grooves such as substantiallycircumferential and axial grooves, that the grooves in thecircumferential configuration fill preferentially. Voids may thus formin the resin filling the axial set of grooves.

The present invention provides an improvement over the apparatusdescribed in U.S. application Ser. No. 619,994 filed Mar. 2, 1967.

This invention is an apparatus for continuously molding thermoplasticresin, comprising structure defining a surface having a pattern thereinand die means having a passage terminating in an outlet positioned inpressure-seal relation with said moving surface for directing moltenthermoplastic resin through said outlet into said pattern, said outletdefined by a downstream surface terminating in a doctor blade and anupstream surface terminating in a zigmg edge having interior andexterior reversals, the interior reversals having an included angle ofbetween 20 and 120 whereby said molten thermoplastic resin is forcedinto said pattern to obtain a continuous web of said moltenthermoplastic resin, means for cooling the web and means for removing;it from said surface. Preferably, the invention is an apparatus forcontinuously molding thermoplastic resin, comprising structure defininga moving surface having a pattern therein, and die means having anoutlet positioned in pressure-seal relation with said moving surface fordirecting molten thermoplastic resin through said outlet into saidpattern, said pattern having grooves running substantiallycircumferentially and substantially axially, said die means having apassage terminating in an outlet positioned in pressure-seal relationwith said moving surface for directing molten thermoplastic resinthrough said outlet into said pattern, said outlet defined by adownstream surface terminating in a doctor blade and an upstream surfaceterminating in a zigzag edge having interior and exterior reversals, theinterior reversals having an included angle of between 20 and 120, theinterior reversal coinciding with a substantially circumferential groovein said pattern of said moving surface, with the pattern of said surfaceforming the base of said passage, whereby molten thermoplastic resin isforced into said pattern to obtain a continuous web of said moltenthermoplastic resin, means for cooling the web and means for removing itfrom said surface. Preferentially the interior reversal has an includedangle of between 60 and Preferentially, the die orifice has awedge-shaped passage which terminates in said outlet and thewedge-shaped passage extends in the direction of movement of thepatterned surface.

These and other embodiments of the present invention will be more fullydescribed in the following detailed discussion and in the accompanyingdrawings in which:

FIG. 1 is a diagrammatic representation of a continuous molding lineincorporating features of this invention;

FIG. 2 shows, in a side elevation cross section, one embodiment ofmolding apparatus for use in the molding line of FIG.

FIG. 3 shows, in a front elevation partial section, the inventivefeature of the present invention;

FIGS. 4A and 4B show, in cross section, across the mouth of thewedge-shaped passage, the zigzag aperture of the present invention.

Referring now to the drawings, in FIG. l is shown an extruder l equippedwith a hopper 2 for receiving thermoplastic resin. The extruder 1 worksthe resin and melts it under pressure. A die 3 receives the pressurizedmolten resin through its rear (hidden) side from the extruder and passesthe resin along a path 4 which terminates in an outlet in pressure-sealrelation with a rotating patterned roll 5 and directs the resinsubstantially free of pressure drop and in the absence of air into thepattern of the roll. The roll 5 continuously moves the molten resin awayfrom the outlet of path 41, thereby forming a continuous molded web 6having a pattern which is complementary to that of the roll. The web 6is chilled by a flume or water spray 11, and after sufficient contactwith the roll 5 which is internally cooled, the cooled web is removedfrom the roll by takeoff rolls 8 aided by a stripper roll 7 and moldrelease agent is applied by spray nozzles 12 to the surface of the rollprior to passage under die 3. Longitudinal dividing or trimming of web 6is accomplished by one or more blades 9 positioned between the takeoffrolls 8 and reel 10.

To further describe the die 3 and patterned roll 5, which comprise themolding apparatus of this invention, FIG. 2 shows one embodiment inwhich die 3 contains a cavity 3' serving path 41 and which is suppliedwith molten ther moplastic resin through inlet by extruder 2 as seen inFIG. 1. Cavity 3 terminates in a slot-shaped outlet 31 extending acrossthe surface of roll 5. The rearward] and forward edges of outlet 31 aredefined by a die plate 33 terminating in a zigzag serrated edge 35, 35and 40, 41 defined in greater detail hereafter and a die plate 32terminating in a doctor blade 34, respectively, each adjustably spacedfrom roll and secured to die 3 by bolts 36 and 36 extending throughslots. The pressure upon the molten resin in the cavity 3' forces theresin through outlet 31 and into the roll pattern represented bytransverse grooves 37 (enlarged in spacing and width for clarity) andintersecting grooves 38 as in FIG. 3. The cavity 3 and path 4 and outlet31 are substantially free of constriction so that the pressure on theresin at the surface of roll 5 is substantially the same as the pressureon the resin in cavity 3.

The grooves 37 (axial) and 38 (circumferential) and protuberences theycreate mold one surface of the web 6. The opposite surface of the web isformed by doctor blade 34 which is adjustably spaced from roll 5 to givethe web thickness desired. Thus, a web consisting of intersectingstrands or ribs corresponding to the grooves 37 and 38 can be obtained,with either apertures being present between the strands or a contourfilm forming one surface of the web. Die 3 may heat to a temperatureabove the resin melting temperature of the particular resin being used,by electrical heating elements extending into corresponding wells in thedie. The resin melting temperature is the minimum temperature at which afresh sample of resin leaves a molten trail as it is moved slowly acrossa heated metal surface. This is also sometimes called the sticktemperature.

Die plate 32 may be heated by an electrical heating element usually to atemperature which is equal to or greater than the temperature maintainedin die 3, thus heating doctor blade 34 to substantially the sametemperature. The downstream face of the die plate terminating in thedoctor blade departs sharply from the path of web 6 so as to avoidsticking of the web to the hot doctor blade. The doctor blade can alsobe notched along its lowermost face to yield corresponding projectionsin the molded web. The upstream surface of die plate 32 may also takethe form of a zigzag which matches the zigzag of die plate 33 edge butis spaced therefrom leaving an outlet having the form of a repeatingpattern zigzag line. Roll 5 is at a temperature which is at least about10 C. less than the melting temperature of the resin being molded. It ismaintained at such a temperature by cooling means such as meansaccommodating a cooling medium passing through an interior passage.

The molding apparatus of FIGS. 2 and 3 can be provided with a waterspray 11 and mold release spray nozzles 12 as shown in FIG. 1.

A pressure-seal relation between the outlet 31 for the moltenthermoplastic resin and roll 5 is maintained so that the pressure on theresin in cavity 3 and the drag flow pressure, is available to force theresin into the pattern of roll 5 on a continuous and high speed ofproduction basis. Generally the resin in cavity 3' is under a pressureof at least 50 p.s.i.g. for resins, such as polyamides. For resins suchas polyethylene, the pressure is generally above 175 p.s.i.g. There is acapability, however, of using much higher molding pressures, such as inexcess of 1,000 p.s.i.g., depending on the pattern being molded. In somecases, drag flow pressure may be used by itself to attain molding. Thepressure-seal relation is obtained, in part, by adjusting the doctorblade 34 to constrict the flow space for the resin as it leaves outlet31 and by having a sufficient rate of web formation for the viscosity ofthe particular resin being molded to prevent backflow under the dieplate 33 which is generally spaced 2 to 10 mils from the surface of roll5.

Means can also be provided for allowing the spacing between the die 3and the roll 5 to change to compensate for pressure fluctuations causedby extruder 1 so as to maintain a constant force on the resin enteringthe roll pattern as described in U.S. application Ser. No. 619,994,filed Mar. 2, 1967.

In this invention, the width of outlet 31 in the direction of rotationof roll 5 should be greater than at least one repeat unit in the patternas described in application Ser. No. 619,994 filed Mar. 2, 1967. Theopening at the end of the die may be wedge-shaped as in FIGS. 2 and 3 orthe die walls at the outlet may be of equal length.

FIG. 3 shows in partial section the serrated edge defined in part byreversals 35, 35 and 41 and edge 40. The serrated edge defines area 39.The outlet 31 through which polymer flows onto roll 5 may be essentiallyof repetitive triangular sections as in FIG. 4A or the outlet may beessentially an open rectangular slot, in which only the upstream edge ofthe outlet is defined by exterior reversals 35, 35 and interior reversal41 and edge 40 as in FIG. 4B.

Serrated upstream edge of die plate 33 and defined reversals may bealmost coterminous with upstream edge of die plate 32, doctor blade 34as in FIG. 4A but need not be so as discussed above and as seen in FIG.4B.

In one embodiment of this invention, the interior reversals 35 and 35are spaced so as to correspond to a groove 38 substantiallycircumferentially aligned in the patterning roll. The number ofcircumferential grooves per reversal of the zigzag upstream edge mayvary from one to 10 or even 20 grooves per reversal depending on thewidth of the grooves and the number of grooves per axial inch of rollsurface. It has been found that the presence of a zigzag pattern in thedie plate 33 provides a more uniform filling of the transverse (axial)grooves of the pattern in roll 5 and more uniform cast product undersome conditions.

Without being bound thereby, it is believed that when the zigzag edge ispositioned as here discussed, it provides for the escape of air from thegrooves. It is believed that the circumferential grooves tend to fillpreferentially and thus trap air in spaces in the transverse groovesproviding resistance to their filling. This difference in resistance tofilling could cause nonuniformities in cast product. Providing interiorreversal in line with circumferential grooves in the roll is believed toprovide preferred paths of escape for this air and thus results are moreuniform filling of the transverse (axial) grooves. The contour of themachined zigzag surface should be in line with normally acceptedextrusion principles.

When a patterned roll having equally spaced axial and circumferentialgrooves on -inch centers and 0.035-inch wide is used, the interiorreversal of the zigzag should be in line with the circumferential groovein the patterning roll. On the other hand, when the grooves are closelyspaced such as 0.020-inch apart, no particular advantage is found inlocating the grooves and interior reversal of the zigzag in anyparticular relationship.

The angle of the reversals 35 and 35 should be between 20 and 120 andpreferably between 60 and Further, it is preferred that both reversals35 35' and 41 be of the same angle. Whereas advantage is gained by thepresence of the zigzag edge, when pattern on the roll gets larger, itbecomes more desirable for the interior reversal to coincide with acircumferential groove in the patterned rolls.

If normally accepted practices are followed, there are no otherlimitations necessary for this invention in the contouring of the zigzagedge.

Web produced according to the present invention can also belongitudinally and transversely drawn to the degree desired as describedin copending application Ser. No. 619,994 filed Mar. 2, 1967.

The shape of the depressions molded into the web can be varied from thecircular form shown to a polygonal form, such as squares, or otherforms, by correspondingly altering the mold pattern in roll 5. Thestrands of the web, whether the web be defilmed, or uniaxially orbiaxially oriented have integrally molded intersections. Some of theuses of screening made according to the present invention are as insectbarriers, lace and other decorative fabrics, buckram, crinoline, andother nonwoven fabrics.

Reinforcing scrim is another use for screening made according to thepresent invention. Reinforcing scrim characteristically has highstrength in one direction but little strength in the transversedirection. Biaxially oriented screening made with the apparatus of thepresent invention overcomes this disadvantage by having high strength inboth directions. The preferred biaxial draw is at least 2.5 times andpreferably at least three times in each direction; high densitypolyethylene screening is preferably biaxially drawn at least four timesthe original screening dimension. A particularly useful biaxially drawnscrim has from two to three strands per inch arranged in a squarepattern and having a strand cross section of about 0.025 inch on a side.Such a product has a breaking strength of about 30 lbs. per inch ofwidth. To maintain good intersection strength of drawn screening, theintersections should include rounded corners, between strands, beforedrawing as well as thereafter.

Typical polymers with which this apparatus is useful are described inUS. application Ser. No. 619,994 filed Mar. 2, 1967.

The particular molding and drawing temperature employed in making andmodifying webs such as hereinbefore described will depend on the resinbeing molded and such operating conditions as the speed of the patternedroll 5 and the intricacy of the pattern therein. The capability of themolding process and apparatus of this invention to mold intricatepatterns is intended to refer to a capability of filling patterns havingclosely spaced grooves and grooves of narrow widths and high depth towidth ratios, as well as other intricacies of pattern design. Thetemperatures will also depend on the nature of the apparatus employed.

In a typical process, an extruder such as described in US. Pat. No.3,325,865 is equipped with a die as described above in pressure-sealrelation to a patterned casting roll. The roll is 4 inches in diameterand o-inches wide. It has a 0.160-inch square pattern of grooves0.035-inch wide by 0.050-inch deep. The edges formed are rounded to0.035-inch radius of curvature.

Nylon resin typically at 280 C. in the die is extruded at 25 feet perminute onto the roll which is maintained at about 1 C. The roll moves ata surface speed of about 40 feet per minute. The roll is initiallysprayed with stearic acid as a release agent, the cavities filled andthereafter water is used to cool the mold, arid the web is removed fromthe roll as in FIG. 1. The die has a zigzag orifice with both theinterior and exteri or reversals equal to 90. When the interiorreversals are aligned with the circumferential grooves on the castingroll, improved filling of the transverse and circumferential groovesresults over that obtained when the same roll is used with a deliverypassage in a slot shape (no zigzag upstream edge).

The same type of process is carried out with a different roll and adifferent extrusion die.

The roll pattern consists of square pegs 0.020 inch on each side rising0.030 inch above the groove bottom (the surface of the roll). Each pegis then rounded to a circular cross section of about between 0020-0023inch in diameter. The die is similar to that used above except thezigzag upstream edge of the die outlet has both interior and exteriorreversals of 60.

Satisfactory product is made with interior reversal of the zigzagaligned with circumferential grooves. Satisfactory product is alsoobtained with the interior reversal of the zigzag edge is aligned withcircumferential grooves.

Typical operating temperatures, including heat stabilization temperaturefor stabilizing oriented web from shrinkage, for some of thethermoplastic resins suitable for use in the present invention are asfollows:

66 nylon 260-350 125-230 I70 frequently called high density polyethylenefrequently called low density polyethylene in the above examples, theincluded angle of reversal 35, 35 and 40 is 60". The depth of groove bythe zigzag from reversal 4l0 to 35, 35 is 0.5 inch. The frequency ofreversal is 1%per inch. There were matching grooves in the upstream dieplate. The peak to peak distance is not critical but is usually between0.25-0.75 inch.

Generally, the passage 31 will take the form of converging surfaces,with the roll pattern forming one of these surfaces. The pressuresrequired on molten thermoplastic resin in cavity 3 can be less than thefull extrusion pressure of the extruder, depending upon which resin isemployed and upon operating conditions. The pressure in the cavity 3however, is substantially the same as the pressure of the resin as itcomes into contact with the pattern of the roll surface. When suchpressure is insufficient, a drag flow arrangement can be used toincrease the force present for continuously filling the pattern withmolten resin. The depth of the grooves forming the pattern in roll 5need not be uniform. Thus, it is apparent that the process and apparatusof this invention is also useful for molding of articles such as are nowinjection molded, with the continuous film produced in the presentinvention resembling the flash obtained in injection molding.

The words circumferential" and axial" are used herein because thepreferred patterned surface is a patterned roll; however, these wordsmean direction of motion of the surface and transverse to thatdirection, respectively. The surface may take the form of a belt carriedby rollers or other mechanical device for transporting resin as is knownin the art so long as the features above described for carrying out theinvention are present.

As many widely different embodiments of this invention may be madewithout departing from the spirit and scope thereof, it is to beunderstood that this invention is not limited to the specificembodiments thereof except as defined in the appended claims.

I claim:

1. Apparatus for continuously molding thermoplastic resin, comprisingstructure defining a moving; surface having a pattern therein and diemeans having a passage terminating in an outlet positioned inpressure-seal relation with said moving surface for directing moltenthermoplastic resin through said outlet into said pattern, said outletdefined by a downstream surface terminating in a doctor blade and anupstream surface terminating in a zigzag edge having interior andexterior reversals, the interior reversals having an included angle ofbetween 20 and whereby said molten thermoplastic resin is forced intosaid pattern to obtain a continuous web of said molten thermoplasticresin, means for cooling the web and means for removing it from saidsurface.

2. Apparatus for continuously molding thermoplastic resin, comprisingstructure defining a moving surface having a pattern therein, and diemeans having an outlet positioned in pressure-seal relation with saidmoving surface for directing molten thermoplastic resin through saidoutlet into said pattern, said surface having at least grooves runningsubstantially circumferentially and transversely therein, said die meanshaving a passage terminating in an outlet positioned in pressuresealrelation with said moving surface for directing molten thermoplasticresin through said outlet into said pattern, said outlet defined by adownstream surface terminating in a doctor blade and an upstream surfaceterminating in a zigzag edge having interior and exterior reversals, theinterior reversals having an included angle of between 20 and 120, atleast some of the interior reversals coinciding with a circumferentialgroove in said pattern of said moving surface, whereby moltenthermoplastic resin is forced into said pattern to obtain a continuousweb of said molten thermoplastic resin, means for cooling the web andmeans for removing it from said'surface.

6. The apparatus of claim 2 wherein the passage is wedgeshaped andextends in the direction of movement of the patterned surface.

7. The apparatus of claim 5 wherein the surface is a roll.

8. The apparatus of claim 6 wherein the surface is a roll.

1. Apparatus for continuously molding thermoplastic resin, comprisingstructure defining a moving surface having a pattern therein and diemeans having a passage terminating in an outlet positioned inpressure-seal relation with said moving surface for directing moltenthermoplastic resin through said outlet into said pattern, said outletdefined by a downstream surface terminating in a doctor blade and anupstream surface terminating in a zigzag edge having interior andexterior reversals, the interior reversals having an included angle ofbetween 20* and 120*, whereby said molten thermoplastic resin is forcedinto said pattern to obtain a continuous web of said moltenthermoplastic resin, means for cooling the web and means for removing itfrom said surface.
 2. Apparatus for continuously molding thermoplasticresin, comprising structure defining a moving surface having a patterntherein, and die means having an outlet positioned in pressure-sealrelation with said moving surface for directing molten thermoplasticresin through said outlet into said pattern, said surface having atleast grooves running substantially circumferentially and transverselytherein, said die means having a passage terminating in an outletpositioned in pressure-seal relation with said moving surface fordirecting molten thermoplastic resin through said outlet into saidpattern, said outlet defined by a downstream surface terminating in adoctor blade and an upstream surface terminating in a zigzag edge havinginterior and exterior reversals, the interior reversals having anincluded angle of between 20* and 120*, at least some of the interiorreversals coinciding with a circumferential groove in said pattern ofsaid moving surface, whereby molten thermoplastic resin is forced intosaid pattern to obtain a continuous web of said molten thermoplasticresin, means for cooling the web and means for removing it from saidsurface.
 3. The apparatus of claim 1 wherein the interior reversal hasan included angle of between 60* and 90*.
 4. The apparatus of claim 2wherein the interior reversal has an included angle of between 60* and90*.
 5. The apparatus of claim 1 wherein the passage is wedge-shaped andextends in the direction of movement of the patterned surface.
 6. Theapparatus of claim 2 wherein the passage is wedge-shaped and extends inthe direction of movement of the patterned surface.
 7. The apparatus ofclaim 5 wherein the surface is a roll.
 8. The apparatus of claim 6wherein the surface is a roll.